A. Field of the Invention
The present invention relates to manually operated relay valves of the type used in fluid control systems such as safety systems for oil and gas wells, and more particularly to a relay valve that is relatively easily manually operable to shut in the system and that must be manually returned to service after the system has been shut in.
B. Description of the Prior Art
Safety systems for shutting in oil and gas well production lines and other flow lines in the event of unusually high or low pressure conditions are well known. Such systems include a pneumatically or hydraulically actuated safety valve for controlling the fluid flow in the line and sensors or pilot valves for sensing pressure in the flow line and producing a signal when the flow line pressure varies from its intended operating range. The systems include a relay valve which receives pressure from the sensors and supplies fluid pressure to the safety valve actuator. When the sensors signal an abnormal flow line pressure, the relay valve operates to vent the safety valve actuator and allow the safety valve to close. Relay valves typically include some means for preventing the relay from going back into service after it has operated to shut in the safety valve. The preventing means may be either external, as shown for example in McMullan U.S. Pat. No. 3,823,739, or internal, as shown for example in Theriot, et al. U.S. Pat. Nos. 3,877,484, or 3,963,050.
A primary shortcoming of the relay valves of the prior art lies in their capacity to be shut in manually. Relay valves include a relatively large diameter pilot piston that is acted upon by pilot pressure to maintain the relay in its in-service position. If the operator desires to shut in the system manually, he must exert a closing force on the relay equal to the pilot pressure multiplied by the area of the pilot piston. Since the safety valve actuator systems operate at relatively high pressures, it has been necessary in the past to regulate and reduce pilot pressure so that the operator can manually shut in the relay. Regulating the pilot pressure has added complexity to the system and has added a source of system failure.
It is therefore an object of the present invention to provide a relay valve that overcomes the shortcomings of the prior art. More particularly, it is an object of the present invention to provide a relay valve that may be manually shut in over a greater range of pilot pressures than was heretofore available. It is a further object of the present invention to provide a relay valve that operates more rapidly to shut in the system in response to a drop in pilot pressure.